In recent years solid state light sources have made their entry as light sources for e.g. video projectors. Some important drivers to replace short arc lamps with solid state alternatives are a long lifetime, avoiding hazardous substances such as mercury, which can be found in some arc lamps, a better reliability and a wider color gamut.
For high brightness levels, e.g. over 2000 lm, the étendue of conventional RGB LED light sources can be too large in order to efficiently couple to small light valves. For high brightness projectors it is therefore common to use RGB (RGB color space) lasers, due to their small étendue which are better suited for small light valves. However lasers can be an expensive solution, in particular green lasers can be expensive and also show lower efficiency. One way of reducing costs of a laser based solution can be to use cheaper types of light sources together with a wavelength conversion technology that can, for example, be based on phosphor or quantum dot luminophores.
With a chosen technology, the short wavelength excitation light, for example blue or near-UV laser light, can be converted into light with longer wavelength, for example green, yellow or red light. An RGB light source may be created, e.g. by utilizing a red laser diode array and a blue laser diode array, together with e.g. a phosphor material. A phosphor material can emit a longer wavelength than it is illuminated with, e.g. it can be illuminated with blue light and emit green or red light. In this way red, green and blue can be obtained with just a red and a blue light source. To facilitate operation, the phosphor can be put on a wheel that can rotate. Such a wheel is hereafter referred to as a wavelength conversion wheel. For single chip projectors, a wheel with different segments of different types of phosphor yielding different colors, can use the light from a blue laser source to sequentially generate all primary colors, such as red, blue and green. Such a wheel having segments of different materials converting the initial light into different colors, is hereafter referred to as a multicolor wavelength conversion wheel.
In order to function with the highest possible effectiveness as a light source, the étendue of the light source should be made smaller than the étendue of the projector. In this way substantially all energy of the light beam will enter the projector. In order to meet this requirement and reduce the étendue, the excitation light source may be limited to illuminating a smaller area (spot size) of the phosphor. However, the smaller the illuminated spot size, the higher the energy density in the corresponding area, and the more difficult it may become to cool the phosphor and maintain the highest conversion efficiency on the phosphor.
In addition to effects related to the étendue, light emitted from a phosphor is not polarized and, thus, phosphor conversion technology cannot be used efficiently in applications requiring polarized light. Such applications can for example be liquid crystal based projectors (e.g. liquid crystal on silicon, or transmissive liquid crystal display technology) and polarized stereoscopic projection.
Solutions to polarize the light from a phosphor source have been proposed in the pending US application US20140240676A1. A wiregrid reflective polarizer is used to reflect the converted light with the unwanted polarization direction back to the phosphor. The power density on the wiregrid reflective polarizer can be high, as the polarizer is in close proximity to the phosphor and subjected to both the incoming excitation light and the converted light. A heat sink for the wire grid can be provided if it is optically bonded to the phosphor layer and the support structure. However, bonding the wire-grid polarizer at the wire-grid side is ruled out since embedding the wire-grid in a medium with a different refractive index would inhibit the wiregrid from proper functioning. Bonding the wire-grid polarizer at the opposite side of the substrate would increase the thermal resistance and the spacing between the wire-grid and the phosphor layer. The increased spacing would lead to widening of light spot size, after one or more reflections, which results in an increase of the étendue of the converted light source.
Polarization recycling is a principle often used for LCD backlights. A reflective polarizer is positioned in between the liquid crystal display and a highly reflective backlight cavity. A special reflective polarizer foil has been described in U.S. Pat. No. 5,486,949 A, using birefringent interference layers, and is commercialized under the name Vikuiti™ Dual Brightness Enhancement Film (DBEF). This type of reflective polarizer has almost no light absorption, and can be cost effectively produced through e.g. by a co-extrusion process, as a thin polymeric foil. A general description of such a foil (or film or sheet) can be a Multilayer Birefringent Interference Polarizer, hereafter referred to as MBIP.
U.S. Pat. No. 7,547,114 B2 discloses a moving plate with wavelength conversion material where a reflective polarizer may be put adjacent to the wavelength conversion material, in combination with an LED or laser light source. However, all the light from the light source with a polarization perpendicular to the transmitting direction of the reflective polarizer will be blocked, hence, it would lead to a severe reduction in energy content. The use of a moving plate complicates cooling and makes fast transitions between color segments (e.g. for color sequential single chip projectors) very hard to achieve.